1. Field of the Invention
The present invention relates to a DC motor, and more particularly to an overload current control circuit for a DC motor.
2. Description of the Prior Art
The characteristic of DC (direct current) is that its working direction is constant, however, the function of a DC motor is to convert electric power into mechanical power, and the DC motor generally comprises a DC generator for converting mechanic power into electric power and a DC motor for converting electric power into mechanical power.
The existing overload current control circuit for a DC motor (as shown in FIG. 1) comprises: a DC motor P10, a DC power source P20, a power semiconductor assembly P30, and a sampling resistor P40 that are connected in series. And the overload current control circuit for a DC motor further comprises an overload protection and detection circuit P50 and a rotation direction switching relay P60.
The overload protection and detection circuit P50 is connected in parallel to the sampling resistor P40 and includes a comparator P51 with one end connected to the power semiconductor assembly P30 and the other end connected to a circuit between the DC motor P10 and the DC power source P20. The rotation direction switching relay P60 is connected in series to a circuit between the power semiconductor assembly P30 and the DC power source P20.
The DC power source P20 supplies power to the DC motor P10, at this moment, the current flows through the sampling resistor P40 to produce a sampling voltage. The comparator P51 of the overload protection and detection circuit P50 can tell if the DC motor P10 is overloaded by detecting the value of the sampling voltage. After the comparator P51 determines whether the current is overloaded, it will send out a LOW signal to disconnect the power semiconductor assembly P30, thus providing a protection function. However, such an overload current control circuit still has the following disadvantages:
The DC power source P20 has positive and negative polarities, when the DC power source P20 changes the polarity of the DC motor P10 by using the rotation direction switching relay P60, the sampling voltages at both ends of the sampling resistor P40 will change polarities alternatively. However, the comparator P51 of the overload protection and detection circuit P50 can only receive single polarity voltage and is unable to change polarity with the change of the polarity of the sampling voltage, so that the overload protection and detection circuit P50 cannot obtain a constant potential sampling voltage to determine overload, causing improper turning on or off of the power semiconductor assembly P30, and as a result, it is unable to provide timely protection to the DC motor P10.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.